There's an simple fitness program called 10,000 steps. The assumption is that the average office worker gets in about 5000 steps a day, but could do with more exercise to get/keep fit. Steps are an easy thing to quantify - you start off small and add a few hundred a week. 10,000 is a nice sounding number, but maybe adding even only 2000, along with watching your diet, would be enough.
So - I splurged and bought a pedometre a few days ago and started off. Lazing around the house, up and down the stairs, a few chores - barely breaks 1000. Taking the bus downtown - rattling your filings - adds 1000 easily as I bounce along. However, I think I'm supposed to turn it off then.
I can head out for a brisk 20 minutes and add 2500-3000 steps, or walk all afternoon and really click on several days worth. The challenge, as with everything, is consistency.